mount.ceph.c: ensure '\0' terminated string

mount.ceph.c: ensure '\0' terminated string

Reserve last char in array for '\0' to ensure termination
of the string.

Fix for:

CID 1128383 (#1 of 1): Buffer not null terminated (BUFFER_SIZE_WARNING)
 buffer_size_warning: Calling strncpy with a maximum size argument
 of 1000 bytes on destination array secret of size 1000 bytes might
 leave the destination string unterminated.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>